Mixing-phase model for shear-induced contractive/dilative effects in unsteady water-sediment mixture flows

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Among the geophysical surface processes, mud and debris flows show one of the most complex and challenging behaviour for scientists and modellers. These flows consist of highly-unsteady gravity-driven movements of water-sediment mixtures with non-Newtonian rheology where the solid concentration could be about 40%–80% of the flow volume and which occur along steep and irregular terrains. Furthermore, the appearance of dynamic pressures in the fluid filling the intergranular pores increases the complexity and dominates the behaviour of the fluidized water-sediment material, leading to the appearance of significant density gradients during the movement. The dynamic pressure in the pore-fluid changes the effective normal stress within the mobilized material, affecting the frictional shear stress between grains and leading to the solid phase dilation/contraction. This must be properly accounted for when developing realistic models for water-sediment surface flows. In this work, a novel physically-based approach for modelling multi-grain dense-packed water-sediment flows is presented. A novel closure formulation for the pressure distribution within the pore-fluid during the movement of dense-packed water-sediment materials has been derived. This closure allows to relate the appearance of shear-induced dynamic pore pressures to the contractive/dilative behaviour of the solid aggregate. The resultant system of depth-averaged conservation laws includes continuity of the density-variable water-sediment material and the different solid classes transported in the flow, as well as the linear momentum equation for the fluidized bulk material, and it is solved using a well-balanced fully-coupled Finite Volume (FV) method. The resultant simulation tool is faced to synthetic, laboratory and real-scale benchmark cases to test its robustness and accuracy. The presence of dynamic pore pressures within the pore-fluid leads to the appearance of a deviatoric contribution to the solid flux, which causes the shear-induced separation of the solid and liquid phases and sustains the flow mobility for long distances, as it has been observed in real mud and debris events.